The present invention relates to a thermal reactor for recombustion of exhaust gas from an internal combustion engine, and more particularly to such a thermal reactor in which heat exchange between the exhaust gas and intake gas mixture takes place through a riser member.
In one prior art system used to reduce the amount of unburnt component of HC and CO in exhaust gas, secondary gas is introduced into the exhaust gas through an air injection manifold provided in the vicinity of and downstream of an exhaust valve and the HC and CO are burnt by the secondary air in a downstream thermal reactor.
However, the above prior art system exhibits certain disadvantages in that (1) when the temperature of the engine is low, the HC and CO are not burnt because the exhaust gas in the reactor is excessively cooled; (2 ) the combustion of the HC and CO does not occur sufficiently because the secondary air and the exhaust gas are not uniformly mixed; and (3) due to the pulsation of exhaust pressure of the exhaust gas, a high pressure is applied to the air injection manifold which tends to block the secondary air jet.
Further, in the above prior art system, an exhaust pipe and an intake pipe are arranged adjacent to each other through a riser member so that heat exchange occurs between the exhaust gas in the exhaust pipe and the intake gas mixture. In such a case, however, when the exhaust gas is at a high temperature the intake gas mixture is excessively heated through the riser member so that the a decrease in the engine output power, and knocking during high speed driving, sometimes occur. An additional disadvantage is that control means for the above prior art heat exchange arrangement is expensive.
Moreover, in the prior art system wherein the intake gas mixture is heated by the exhaust gas which is re-burnt in the thermal reactor, there are still further disadvantages in that the intake gas mixture may not be heated when the temperature of the engine is low because the gas in the thermal reactor is cooled by the secondary air, and when there is active combustion of the HC and CO in the thermal reactor the intake gas mixture is excessively heated and the carburetor is also heated by thermal conduction resulting in vapor locking.